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I. Factors causing deformation during heat treatment 1. Carbon content and its effect on the degree of change during heat treatment: As the yield strength of high-carbon steel increases, its degree of deformation is less than that of medium-carbon steel. For carbon steel, in most cases, the deformation amount of T7A steel is the smallest. When the mass fraction of carbon is greater than 0.7%, it tends to shrink ； However, when the carbon mass fraction is less than 0.7%, both the inner diameter and the outer diameter tend to expand. Carbon steel has a relatively low yield strength; as a result, carbon steel components with internal holes (or cavities) experience significant deformation, causing the internal holes (or cavities) to expand. Due to its high strength, low Ms point, and large amount of residual austenite, alloy steel experiences less distortion during quenching; the deformation that occurs is mainly of the thermal stress type, resulting in a reduction in the inner diameter (or cavity size) of the steel parts. Therefore, when quenched under the same conditions as medium-carbon steel, high-carbon and high-alloy steel workpieces tend to experience shrinkage primarily in their inner holes. 2. Influence of alloying elements on heat treatment deformation: The effect of alloying elements on the heat treatment deformation of workpieces is primarily reflected in their impact on the Ms point and hardenability of steel. Most alloying elements, such as manganese, chromium, silicon, nickel, molybdenum, boron, etc., lower the Ms point of steel and increase the amount of residual austenite. This reduces the change in specific volume and the structural stress that occur during steel quenching, thereby minimizing the quenching deformation of the workpiece. Alloying elements significantly increase the hardenability of steel, thereby enhancing its volumetric deformation and structural stress, which leads to an increased tendency for deformation during heat treatment of the workpiece. Furthermore, since alloying elements increase the hardenability of steel and reduce the critical quenching cooling rate, in actual production a milder quenching medium can be used, thereby reducing thermal stress and minimizing the thermal treatment deformation of the workpiece. Silicon has little effect on the Ms point; it only serves to reduce the deformation of the specimen ； Tungsten and vanadium also have little effect on hardenability and the Ms point, and they cause minimal deformation during the heat treatment of workpieces. Therefore, the so-called micro-deformed steels used in industry all contain relatively large amounts of alloying elements such as silicon, tungsten, and vanadium. 3. Influence of the original microstructure and stress state on heat treatment deformation: The original microstructure of the workpiece prior to quenching, such as the shape, size, quantity, and distribution of carbides, as well as the segregation of alloying elements and the fiber orientation resulting from forging and rolling, all have an impact on the heat treatment deformation of the workpiece. Spherical pearlite has a larger specific volume and higher strength than flaky pearlite; therefore, workpieces that have undergone pre-spheroidization experience relatively less quenching deformation. For some high-carbon alloy tool steels, such as 9Mn2V, CrWMn, and GCr15 steels, the spheroidization grade has a significant impact on heat treatment-induced deformation and cracking, as well as on the correction of deformation after quenching; generally, a spheroidized structure at grade 2.5–5 is appropriate. Quenching and tempering not only reduces the absolute value of the workpiece’s deformation but also makes the quenching-induced deformation more regular, thereby facilitating control over the deformation. The distribution of strip-like carbides has a significant impact on the heat treatment deformation of the workpiece. After quenching, the workpiece expands in the direction parallel to the carbide bands, and contracts in the direction perpendicular to them; the larger the carbide particles, the greater the expansion in the direction of the bands. For ledeburite steels such as Cr12-type steel and high-speed steel, the morphology and distribution of carbides have a particularly significant impact on quenching deformation. In short, the more uniform the original microstructure of the workpiece, the less deformation occurs during heat treatment; the deformation is more regular, and it is easier to control. 4. The stress state of the workpiece itself before quenching has a significant impact on deformation. This is especially true for workpieces with complex shapes that have been machined using large feed rates; if the residual stresses in such workpieces are not eliminated, they can have a substantial effect on quenching-induced deformation. 5. Influence of the workpiece’s geometric shape on heat treatment deformation: For workpieces with complex geometries and asymmetric cross-sectional shapes, such as shafts with keyways, tools used for creating keyways, and tower-shaped components, during quenching and cooling, one side cools down rapidly while the other side cools down more slowly, resulting in uneven cooling. If the deformation caused by uneven cooling above Ms dominates, the side that cools faster will be concave; whereas if the deformation caused by uneven cooling below Ms dominates, the side that cools faster will be convex. Increasing the isothermal treatment time increases the amount of bainite transformation, making the residual austenite more stable and reducing the amount of martensite transformation during air cooling, which can significantly reduce the deformation of the workpiece. 6. Influence of process parameters on heat treatment deformation: Both conventional and special heat treatments can result in heat treatment deformation. When analyzing the impact of heat treatment process parameters on this deformation, it is most important to examine the effects of the heating and cooling processes. The main parameters of the heating process are heating uniformity, heating temperature, and heating rate. The main parameters of the cooling process are cooling uniformity and cooling rate. The effect of uneven cooling on quenching deformation is the same as that caused by uneven cooling due to the asymmetrical shape of the workpiece cross-section; this section mainly discusses the influence of other process parameters. II. Methods for preventing and controlling deformation due to heat treatment 1) Reverse bending method: Based on the patterns of deformation that occur in shaft components during heat treatment, a stress can be applied in advance before quenching – specifically, by bending the component in the opposite direction to that in which deformation would occur – thereby compensating for the bending deformation that results after quenching and reducing the amount of work required for straightening. Suitable for parts with significantly uneven cross-sections and severe deformation. 2) Static quenching method: It requires that the temperature of the quenching coolant be uniform, and it should be in a static state right after being stirred just before quenching. Clamp the part with pliers and plunge it into the cooling liquid. This method results in a much smaller degree of deformation upon quenching compared to using wire suspension. 3) Uniform and symmetrical part design: The cross-sectional shape of the part should be designed to be as uniform and symmetrical as possible; process grooves can be created if necessary. For example, the boring bar has two symmetrical grooves; in practice only one is used, while the other is designed to reduce thermal treatment-induced deformation. 4) Quenching using a dedicated quenching fixture: If the cross-section of the part is symmetrical, it can be placed into a specialized fixture after being taken out of the furnace, and then quenched in cooling fluid in a vertical direction. Since part deformation is restricted by the fixture, it can generally be kept within the allowable tolerance range. 5) Heating using an embedded salt bath furnace: With an insert-type salt bath furnace, heat is applied to only one side of the part, which can lead to bending and deformation; in contrast, an embedded salt bath furnace provides more uniform temperature distribution and is energy-efficient. A fluidized particle furnace can also be used. 6) Vertical lifting and horizontal placement: When storing long parts before and after quenching, care should be taken to prevent the parts from bending due to their own weight; it is best to use racks for vertical lifting. For long-distance transportation, multiple plastic air cushion packs can be used; this not only allows the parts to achieve automatic balance but also provides shock absorption. 7) Stress relief before quenching: Used for important parts that are prone to deformation, such as precision long screw rods. Annealing or normalizing is performed before quenching to refine the grain structure and homogenize the microstructure, thereby reducing internal stresses. During quenching heating, the heating temperature must be strictly controlled. III. Machining correction methods for heat treatment deformation. Taking a carburized and quenched gear shaft as an example, the machining allowances before carburizing and after quenching are quite large according to the standards; grinding it directly is neither economical nor convenient. Given its high hardness, what methods exist for processing it directly? By using the non-metallic adhesive HLCBN tool of BN-S20 grade, it is possible to complete the machining of hardened workpieces in a single pass with a large amount of material removed. The superhard tool of BN-S20 grade can remove a large portion of the hardened layer, thus replacing the need for rough grinding with large material removal; this eliminates the traditional processing method that involves annealing followed by secondary quenching, saving on processing costs and related operational expenses while significantly improving production efficiency. TOOLOX, the revolutionary new concept of pre-hardened tool steel produced by the globally renowned Swedish SSAB steel company, is manufactured using the CSR smelting process, which ensures extremely high purity as well as very low levels of S and P (0.008 and 0.002 respectively). It benefits from world-leading heat treatment techniques (1000°C/second), resulting in minimal residual austenite. The steel is delivered in a pre-hardened state, with a maximum hardness of 48HRC (or even higher); it is thus the hardest pre-hardened tool steel in the world. Despite such high hardness, it remains easy to machine. TOOLOX represents the perfect combination of high hardness and toughness, making it the ideal choice for improving production efficiency. It can be processed without the need for heat treatment, thereby **reducing production cycles and associated heat treatment risks** ； Its ultra-high purity and minimal residual stress result in excellent stability of the steel, with very little dimensional deformation and outstanding high-temperature performance. It is widely used in precision molds, precision mechanical parts, shock-resistant cutting tools (tool handles, tool shanks, cutter heads), fixtures and gauges, as well as high-temperature resistant components, especially in those applications where strict requirements are placed on dimensional accuracy.